For me it is convenience as tooling like fmt, testing, etc. are build in and they try to keep api close to browser api's when possible.
I've no usage experience myself, but, I'd say running (and now packaging or compiling) modern JS or Typescript directly without having to build a pipeline with tools like webpack.
I don't think Deno's gonna help deal with something like Webpack, is it?
Webpack's for building browser bundles, Deno's about running TypeScript server-side, isn't it?